The field of the invention relates to flame supporting elements for burners, notably premixing burners fed with a combustible gas.
The Prior Art suggests various types of flame supporting elements for stabilizing the flames and for improving the development of said flames.
Such flame supporting elements are designated by other expressions such as  less than  less than burner membrane greater than  greater than ,  less than  less than porous metal fiber plate greater than  greater than  or  less than  less than combustion head greater than  greater than .
The flame supporting elements are typically made of materials such as ceramic or metal. They are porous or have holes therein for enabling the gas to pass therethrough.
In the burners, the flame supporting elements are typically interposed between the distribution chamber and the combustion chamber.
In U.S. Pat. No. 3,680,183, is disclosed a process for manufacturing such a flame supporting element for burner.
According to said process:
a) distinct metallic fibers are made from a metallic alloy heatproof up to at least substantially 750xc2x0 C. and comprising iron, chromium and aluminum,
b) the distinct metallic fibers are joined to each other, under a determined pressure, for creating a mat of agglomerated fibers, and
c) the mat of agglomerated fibers is heated to a temperature which is sufficient for intimely joining the agglomerated fibers forming the mat, at zones where said fibers intersect.
Even if the disclosure of the above US patent relates to a burner, it does not specifically concern a gas burner. And various drawbacks are considered in the invention as being to be absolutely solved, as regards the state of the Art.
So, an object of the invention is to provide gas burners with a flame supporting element fulfilling the following features:
a flame support element adapted for operating as in a  less than  less than blue flame greater than  greater than  mode (the flames are typically located outside the flame supporting element) as in a radiant mode (the flames penetrate within the flame supporting element),
easiness and swiftness for manufacturing the flame support element,
reliability of the flame supporting element (resistance to oxidation, mechanical strength, emission of pollution, variability of power the modulation of the power should reach 1 to 10, even 1 to 30),
quality of the flame supporting element as manufactured, especially in consideration of the mechanical features and elasticity,
cost price as low as possible,
variability in the shape of the flame supporting element.
The solution as proposed by the invention for satisfying at least some of the above features, consists in:
during the above step a),
providing a tank containing a metallic alloy having an aluminum content higher than substantially 4% in weight (or even 5%), the tank being heated up to a temperature higher or equal to the temperature for fusing said metallic alloy,
setting a contact between said fused metallic alloy and movable extracting means, so that the movement of the extracting means induces a determined quantity of said fused (melted) metallic alloy to adhere the surface thereof, to be extracted from the tank, then to be cooled, for solidification, at first on said surface of the extracting means, then in air (or in a neutral gas), when said quantity of extracted metallic alloy has left the surface of the extracting means under the effect of a separating force induced by the movement of the extracting means,
during step b), the distinct metallic fibers (which are typically still individualized and which are in a dry state, as obtained from the step a)) are disposed in a mold where they are substantially uniformely compressed for forming said agglomerated mat, in such a way that the gas porosity in the mat is substantially uniform,
and, during step c),
no pressure notably higher than the pressure exerted during step b) is exerted to the compressed fibers of said agglomerated mat,
the mat of the compressed fibers is connected to electrodes and to a capacitor,
and through said electrodes and by electrically discharging the capacitor, the compressed fibers of the mat are heated at the points where they are in electrical contact to each other, to a temperature higher or equal to their temperature of fusing, for inducing a welding of the fibers to each other, exclusively, under a  less than  less than high voltage greater than  greater than  (at least substantially 1000 Volts), so that the gaseous porosity of the mat comprising said metallic fibers welded to each other is substantially uniform and substantially equal to the gas porosity of the agglomerated mat obtained from step b).
By using such a process:
the steps for manufacturing the mat of fibers are reduced (only one step is useful for manufacturing the mat of compressed fibers, by using  less than  less than dry greater than  greater than  metallic fibers originally distinct one from the other),
a mat having high thermical and mechanical performances is obtained,
during step a), valuable metallic fibers are manufactured, and during the further steps of manufacturing the thermical and mechanical performances of such fibers are maintained, with no alteration of those performances during the step of compressing or the step of intimely mechanically connecting the fibers to each other,
the manufactured flame support element has an homogeneous gas porosity, what is in favor of the optimization of the gas burner,
the mechanical strength of the flame supporting element is improved.
It will also be noted that the above-mentioned expression  less than  less than (welding) greater than  greater than  (the agglomerated fibers of the mat are welded to each other) specifically relates to welding said fibers exclusively one to the others, at a temperature at least equal to their temperature of fusing, what is really different from sintering the fibers. Further, such a  less than  less than welding greater than  greater than  is, in the present case, more specifically a welding induced by electrically discharging a capacitor, what is completely different from a welding obtained by using a welding machine comprising an electrical transformer which delivers an electrical voltage very lower than the above-mentioned  less than  less than high voltage greater than  greater than  (a few dozens to a few hundreds of volts, only, what is not presently appropriate, as regards the required mechanical and thermical features, together with the performance of a qualified flame supporting element in a gas burner).
According to the invention, the welding of the metallic fibers is operated at a voltage of at least 1000 Volts (typically a few thousands, or even a few tens of thousands, of Volts) under an intensity of at least 100 A (the intensity can be higher than 10 000 A), during a period of time of about 10 to 20 micro-seconds.
It is further to be noted that another feature of the invention preferably requires that during the step a), the metallic fibers as manufactured contain preferably between 5.5 and 8% in weight of aluminum.
For favorizing the circulation of the gaseous fluid through the flame support element, the fibers as obtained from step a) will preferably be elongated fibers showing a transversal section having a shape of lunule (viz. lenticular or  less than  less than crescent  greater than  greater than -shaped) such a shape defining a hollow canal (on the concave face).
Along the transversal section, the outer cord of such fibers will preferably be comprised between 300 microns and 3000 microns (average value typically about 800 xcexcm), and an average height of about 20 to 200 xcexcm. The length of the fibers will preferably be comprised between 0.7 cm and 15 cm, and advantageously higher than 1 cm. As regards the gas porosity of the flame supporting element, said porosity will preferably be comprised between substantially 60% and 95%. Metallic fibers will then be preferably isotropically dispersed in the mat, and the flame supporting element will be adapted to be used, as in an air atmospheric burner, as in a pressurized air burner.
For manufacturing the above-mentioned metallic fibers, the above-mentioned  less than  less than extracting means greater than  greater than  preferably comprise a wheel having a peripheral surface provided with grooves (or indentations) regularly spaced and individually provided with a thin line. The wheel is rotated and the thin line of every groove is brought substantially to the same level than the melted metal, so that every groove extracts a determined quantity of metallic alloy, said quantity being substantially equal to the quantity useful for manufacturing the metallic fibers, once the metal is cooled and solidified.
It is also to be noted that the conditions of molding the fibers under pressure and/or welding them one to the other will be different, as a function of the gaseous porosity of the flame supporting element: if the gas porosity is comprised between about 60% and 80% to 85%, then, the molding under pressure will be operated within the mold. However, the welding operation will operate out of the mold (the walls of the welding apparatus will then be electrically insulated ; the electrodes only will be electrically conductive).
The heating temperature at the points (or zones) where the fibers are in contact to each other will typically reach, or be higher than, 1450xc2x0 C.
If the gas porosity is higher than above (about 85% to 95%), both the molding and the welding operation will operate within the molding apparatus (still having a non electrically conductive wall). The temperature will substantially be equal to the above-mentioned one.